A 1-D marine stratocumulus cloud model has been supplemented with a comprehensive and up-to-date aqueous phase chemical mechanism for the purpose of assessing the impact that the presence of clouds has on gas phaseHO<sub>x</sub>, NO<sub>x</sub> and O<sub>3</sub> budgets in the marine boundary layer. The simulations presented here indicate that cloud may act as a heterogeneous source of HONO<sub>g</sub>. The conversion of HNO<sub>4(g)</sub> at moderate pH (~ 4.5) is responsible for this, and, to a lesser extent, the photolysis of nitrate (NO<sub>3</sub><sup>-</sup>). The effect of introducing deliquescent aerosol on the simulated increase of HONO<sub>g</sub> is negligible. The most important consequences of this elevation in HONO<sub>g</sub> are that, in the presence of cloud, gas phase concentrations of NO<sub>x</sub> species increase by a factor of 2, which minimises the simulated decrease in O<sub>3(g)</sub>, and results in a regeneration of OH<sub>g</sub>. This partly compensates for the removal of OH<sub>g</sub> by direct phase transfer into the cloud and may have important implications regarding the oxidising capacity of the marine boundary layer.